Conventional curtains to be attached to a window side of a room are used solely for the specific purposes of providing an ornament for a room and obtaining an internal space of a room or securing privacy.
In ordinary practice, two curtain rails are employed and a single sheet of curtain made of a relatively thick woven fabric or the like is hung on the room side or the window side and another single sheet of curtain made of a relatively thin material such as a lace material is hung on the window side or the room side, so that they can be opened and closed in accordance with necessity. In a day time, the relatively thick curtain made of a woven fabric material or the like is pulled opposite sides of the window in order to permit sun light to enter the room and only the relatively thick curtain made of a lace material or the like is closed. At night, however, only the relatively thin curtain made of a lace material or the like is not good enough because the inside of the room can be seen from outside through the lace curtain under the effect of light of the room lamp. Therefore, it is necessary to close the relatively thick curtain made of a woven fabric material so that the inside of the room cannot be see from outside.
The curtain used in the manner as mentioned above is called "dual curtain". As mentioned above, it is necessary for such a dual curtain to employ two curtain rails and two sheets of different kind of relatively thick and thin curtain materials.
In case of a window provided with only one curtain rail by some reasons (for example, a limited space just enough for mounting only one curtain rail, an economical reason or the like), only one of the curtain sheets, either the relatively thick one or the thin one, is employed. In this case, it is impossible, unlike in the case of the dual curtain, to selectively use the relatively thick or thin curtain in accordance with necessity.
There is also another example of a conventional curtain formed by integrally attaching two layers of fabric materials together. In this example, a sheet of front curtain material (the fabric appeared to the room side) is attached with a sheet of back curtain material (the fabric appeared to the window side) so that light shieldability is enhanced. There is still another example of a conventional curtain formed by integrally attaching two layers of fabric materials together. In this example, two sheets of relatively thin curtain material each having a good light permeability are attached together so that the final curtain allows light to permeate therethrough but does not allow the inside of the room to be seen from outside.
In the above additional examples, both the front and back curtain material sheets are attached by stitching with lug portions on opposite sides and occasionally with skirt portions and the two sheets of front and back curtain material are usually used as one piece. However, since there are differences in kind of the starting yarn used in the two sheets of fabric material (front sheet and back sheet), weaving texture, amounts of finishing agents such as various kinds of resins at the time of processing, heat effect at the time of heat set, etc., the two sheets of fabric material are not always entirely equal in elasticity.
Accordingly, if those two sheets of fabric material are stitched at their lug portions at opposite sides thereof into an integral curtain, a contracting wrinkle and an expanding wrinkle tend to occur in use due to difference in elasticity between the front fabric material and the back fabric material, thus resulting in a poor appearance.